General Nonimmigrant Visa

I recently moved from Company A to B on July-2016 as a contractor while on i was on my 6th year H-1B visa. Company B did my H-1B transfer and I-140 got approved through them as well. Now working at End client location placed through Company B. Now End client wants to hire me as Full Time employee.<br>
1. Given I recently joined Company B will my jumping to Company C cause any issue to my H-1B visa? <br>
2. Can company B cancel my H1B petition?If Yes, Do i have to have a approved H-1B petition from Company C before informing Company B about the offer?<br>
3. Will my I 140 which got approved thru company B still be valid? Can i use it for future extensions of H-1B?<br>
4. Company B did not provide I-140 copy, how to obtain the same from USCIS?<br>
5. Is there any mandatory period to serve in one company who has sponsored my GC as I recently got my I-140 approved in August-2016?

Can I apply for STEM OPT, if I work for a E-Verified staffing agency (similar to Aerotek, Kelly services) on contract basis that is related to my STEM field of education at a client location ?. (I along with staffing agency will fill out I983 form) <br>

I read online that USCIS has regulated "Certain Types of Employment" for STEM OPT in this new rule. So am worried that working for a staffing company at a client company location is therefore invalid :/ ((If so, I think then all the staffing companies would suffer as well))<br>

I have attached hereby the snippet from Federal Register:<br>

""""""There are several aspects of the STEM OPT extension that do not make it apt for certain types of arrangements, including multiple employer arrangements, sole proprietorships, employment through “temp” agencies, employment through consulting firm arrangements that provide labor for hire, and other relationships that do not constitute a bona fide employer-employee relationship. One concern arises from the difficulty individuals employed through such arrangements would face in complying with, among other things, the training plan requirements of this rule. Another concern is the potential for visa fraud arising from such arrangements. Furthermore, evaluating the merits of such arrangements would be difficult and create additional burdens for DSOs. Accordingly, DHS clarifies that students cannot qualify for STEM OPT extensions unless they will be bona fide employees of the employer signing the Training Plan, and the employer that signs the Training Plan must be the same entity that employs the student and provides the practical training experience. DHS recognizes that this outcome is a departure from SEVP's April 23, 2010 Policy Guidance (1004-03)."""""
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It would be really helpful if you could explain what the above paragraph means.

I have PERM approved and I-140 filed in regular processing by my current employer "A". I have got an opportunity from employer "B" and then told they will process my GC as fresh application. I have 17 months left on my H-1B visa.
1. In pending approval of I-140. will I get the extension of 3 years from USCIS when employer "B" initiate the transfer ? OR Will I get the H-1B transfer extension till max out ?
2. Is it recommendable to switch now in between of pending approval process of I-140?
3. Will leaving in between of pending process impact me in future when employer "B" files my GC once again ?